This invention relates to planarizing techniques for use in photolithographic processing of integrated circuits. More particularly, the invention provides a method of forming a desired photolighographic pattern of silylated planarizing resist on an organic planarizing layer to create high-resolution patterns on a substrate.
In a typical integrated circuit device, electronic circuits are fabricated on semiconductor substrates with the components of individual active devices separated by layers of insulation and interconnected via conductive layers. Complex procedures are required to manufacture such circuitry because the surface topography of the substrate must be changed and refined at numerous stages in the process. Adding and removing precisely defined surface features is accomplished using photolithography, which requires the creation of detailed mask patterns which define certain areas that are subsequently removed by chemical etching.
Complex integrated circuits, manufactured to include high density concentrations of electronic components, require the application and removal of numerous photolithographic masks during the processing of the substrate. Creating fine distinctions in surface topography during the latter stages of device manufacturing is particularly difficult. Increasing circuit complexity and miniaturization requires manufacturers to continually develop improved photolithographic methods for creating high definition masks.
One type of photolithographic processing known generally as planarizing employs multilayer resists to achieve the fine definition masks required for Very Large Scale Integration (VLSI) chips having in excess of one hundred thousand components per chip. Planarizing techniques generally involve coating the topography of a semiconductor wafer substrate with a planarizing layer of photoresist and then forming a photolithographic mask in the photoresist using a second or third masking layer.
One planarizing technique used to create high definition masks is termed the bi-layer technique. It uses two layers of photoresist to resolve small geometry's on wafers with varied topographies. Typically, prior art bi-layer planarizing uses two organic layers, a bottom or "planarizing" layer and a top, imaging resist layer in which a mask pattern is created. The two layers have different chemistries, which allows them to be separately etched. The top imaging layer can thus be used as a mask to etch and configure the planarizing layer. A serious limitation of bi-layer planarizing is the need to use planarizing compositions which are sufficiently different from the imaging resist to avoid interfacial mixing. It also requires different irradiation wavelength absorption characteristics in the two layers and different photolithographic processing so the layers can be resolved separately. These factors conspire to limit the planarizing compositions to materials that are much more difficult to process than conventional resists. Consequently, bi-layer planarizing is complex to process, tends to have high defect levels, and is used only for limited, low-volume applications.
Another prior art planarizing technique which improves upon the bi-layer process is referred to as tri-layer planarizing. The tri-layer process incorporates a "hard" layer between the two resist layers of the bi-layer process. The hard layer, in most prior art applications, is a deposited layer of silicon dioxide or other developer-resistant material. Essentially, the tri-layer system deposits a bottom planarizing layer on the substrate topography, then deposits the "hard" layer on the planarizing layer, and finally deposits an imaging resist on the hard layer. The patterned image is formed on the top imaging resist, which is subsequently etched into the hard layer and finally into the planarizing resist. Use of the hard layer simplifies photolithographic processing because it separates the two resist layers and allows each to be processed separately, using similar or identical etch chemistries.
The disadvantage of prior art tri-layer planarizing processes is that the hard layer is difficult to work with. A typical hard planarizing layer used in prior art systems is spin-on-glass ("SOG"). The "glass" is a mixture of silicon dioxide and a solvent that evaporates quickly. After the planarizing resist layer is deposited, the SOG is spun onto the planarizing layer and the glass film is heated to help evaporate the solvent. The result, ideally, is a layer of silicon dioxide film. Unfortunately, the SOG technique produces high defect densities due to particulate formation and deposition during the evaporation of the isopropyl alcohol ("IPA") solvent used in SOG. One mechanism which causes defects is the agglomeration of silicon dioxide into particles in the silicon dioxide-IPA mixture itself. Particles also form during the spin-on application procedure since the IPA evaporates rapidly and can leave encrusted particles of silicon dioxide at the tips of the dispense lines. By whatever method, defect-causing particles tend to be formed and deposited as part of the SOG layer. Despite its deficiencies, the tri-layer technique is an improvement over the bi-layer methodology due to the ease of etching the SOG and planarizing layers with a single step. Tri-layer planarizing tends to produce a highly accurate photolithographic pattern in the planarizing layer.
Yet another prior art planarizing technique involves the incorporation of silicon into selected areas of the planarizing photoresist to create a pattern of "hard" surface regions on the resist. One such technique is disclosed in U.S. Pat. No. 4,882,008. In that patent one or more layers of resist are applied to the substrate. An imaging resist is then applied, defining a photolithographic pattern on its surface. The resist is then exposed to radiation and then to a gaseous silicon-containing species such as hexamethyldisilazane. Silicon is thereby incorporated into the irradiated regions of the resist. The silicon-enriched regions are subsequently exposed to an oxygen plasma which converts the silicon to silicon dioxide. The result is a pattern of silicon dioxide formed in the planarizing photoresist adjacent the surface of the resist. The silicon dioxide serves as a "hard" mask for subsequent photolithographic processing of the photoresist. A similar process is discussed in U.S. Pat. No. 4,963,463, which discloses a photoresist resin composition which forms an acid when exposed to an appropriate wavelength of radiation. The radiation-sensitive resin in the '463 patent may be based on a condensate of an alkali-soluble resin such a novolac quinonediazide compound. The '463 patent describes a process in which selected areas of the photoresist are exposed to radiation and treated with a silicon-containing compound, which causes the silicon to incorporate into the resist layer, in the selected areas, by reaction with the acid.
A major problem with the techniques disclosed in the '008 and '463 patents is the poor-quality "hard" mask patterns which result from the disclosed methodologies. In both patents, the photoresist layer is exposed to radiation only in selected areas through a pre-applied mask. As a result, only selected areas are irradiated and processed to incorporate silicon into the resist composition. Unfortunately, the silicon dioxide pattern which results from such patterned irradiation turns out to be non-uniform in depth and composition, resulting in non-uniform edge lines. For example, in FIG. 3e of U.S. Pat. No. 4,882,008, a uniform-depth, thin layer of silicon dioxide 158 is illustrated as having been deposited precisely over the patterned area of photoresist 155. In reality, the silicon dioxide layer will turn out to be thick in the center and will taper to nothing at the side edges. That is believed to result from non-uniform incorporation of silicon into the photoresist through the mask openings. In other words, the pattern of silicon dioxide deposited is uneven, with substantial gradients in thickness. As a result, the silicon dioxide will have edge lines which thin to zero thickness, causing the edge lines on the mask pattern to be uneven and to have poor masking qualities. Consequently, fine etch lines cannot be made in the underlying resist.
Yet another tri-layer planarizing technique, representing a substantial advance in the tri-layer methodology, is disclosed in copending patent application Ser. No. 07/893,702, filed Jun. 5, 1992, entitled "Silylated Photoresist Layer and Planarizing Method," assigned to the same assignee as the present invention, of which the present application is a continuation-in-part. In application Ser. No. 07/893,702 a planarizing method and composition is disclosed which eliminates the need to use the defect-prone SOG material. Instead, the organic planarizing layer applied to the surface topography of a wafer substrate incorporates a suitable acid in its composition which allows, through careful processing, a blanket surface film of silicon dioxide to be formed in the planarizing layer. A photolithographic pattern is then formed in the film of silicon dioxide through application of a patterned imaging resist followed by suitable photolithographic etching steps. The silicon dioxide pattern is used as a mask for further processing of the planarizing resist. The present invention is a modification of the method and composition disclosed in application Ser. No. 07/893,702, the specification of which is incorporated herein by reference.
It would be advantageous to provide an alternative methodology for producing high-definition silicon dioxide mask patterns on the surface of a planarizing photoresist, without the disadvantages of using SOG.
It would also be advantageous to provide a simplified tri-layer planarizing technique which improves the sharpness of the mask pattern without the complexity and high defect densities of conventional prior art tri-layer techniques.
It would also be advantageous to provide a method for producing a mask pattern of silicon dioxide on the surface of a planarizing photoresist layer using a methodology which greatly improves the sharpness of the patterned edges over those produced by prior art methodologies.
Accordingly, the present invention provides a method of forming a mask pattern of silicon dioxide on the surface of a layer of photoresist. The steps in the method include irradiating the surface of the photoresist with a substantially uniform, unpatterned, shallow-penetrating radiation to create a substantially uniform, unpatterned, silicon-reactive region adjacent the surface of the photoresist. The next step is to softbake the irradiated photoresist in a silicon-containing environment to convert the silicon-reactive region to a silicon-enriched region adjacent the surface of the photoresist. The next step is to form a desired photolithographic pattern of imaging resist on the surface of the photoresist, overlying the silicon-enriched region. The next step is to form the photolithographic pattern in the silicon-enriched region of the photoresist using the imaging resist as a mask. The next step is to etch the resultant structure using an oxygen plasma to remove the imaging resist and convert the remaining portions of the silicon-enriched region to silicon dioxide. The result is a high-definition mask pattern of silicon dioxide on the surface of the photoresist.
In order accomplish the shallow-penetration irradiation step, the invention preferably employs radiation having a wavelength generally in the range of between 200-nanometers and 320-nanometers. The radiation will penetrate the surface of the photoresist to a depth of generally 3,000 .ANG. or less. The preferred planarizing photoresist composition includes one or more polymers selected from the group consisting of a novolac, polymethylmephacrylate, polydimethylgluparimide, and a polhydroxystyrene.
The method of the present invention can produce high-resolution patterns on a substrate. It preferably includes a preliminary step of applying the planarizing photoresist composition to the surface topography of the substrate. The irradiating and baking steps then follow the step of applying the planarizing photoresist. The step of forming the desired photolithographic patten of imaging resist more specifically includes coating the planarizing photoresist with a layer of imaging resist and then applying conventional photolithographic and etching steps to create the desired patten in the imaging resist. The step of forming the photolithographic patten in the silicon-enriched region preferably involves etching the planarizing photoresist using the imaging resist as a mask, with the etching being carried sufficiently far into the photoresist to remove the silicon-enriched layer near the surface in the selected areas being etched. The step of etching the resultant structure using an oxygen plasma accomplishes the removal of the imaging resist, and converts the silicon-enriched layer to silicon dioxide. It also removes the remaining planarizing photoresist in the regions defined by the photolithographic pattern, with the silicon dioxide serving as a mask. As a result, a high-definition photolithographic pattern is created on the planarizing photoresist topography of the substrate.